The need for improved low-cost protection of coaxial transmission lines from lightening strikes and induced power surges has emerged in the communication technology coincident with the proliferation of coaxial systems in cellular telephone and coaxial usage for broadband transmission. Coaxial transmission is characterized as a continuous path capable of directing the transmission of electromagnetic energy along its path. If the geometrical dimensions and the constants of the materials are identical, the line is said to be uniform. Any changes in the physical configuration of the transmission line are said to be nonuniformities or discontinuities in the system.
Although many types of lightening protectors are manufactured and sold for use in coaxial systems for lightening protection, they all have a common deficiency in that they all create a discontinuity in the path of the coaxial line. The common feature of all existing protectors is that the protecting element is connected between the center conductor of the coax and the outside sheath. In effect, the protecting element is placed in shunt with the transmission line. This placement creates a nonuniformity which results in a higher insertion loss and also a higher reflection loss to the electromagnetic waves which are propagated on the line.
Existing protectors have attempted to compensate for the discontinuities by placement of shunt voltage limiters by modifying either the inner or outer diameter of the line to obtain the equivalent of the characteristic impedance of the line. This procedure has been successful only for limited regions of the bandwidth. The compensations provide acceptable transmission for limited bandwidths but not for the full spectrum bandwidth capability of the line. In today's market, the crowded electromagnetic spectrum has necessitated the efficient and complete usage of all available bandwidth.
A device as shown in U.S. Pat. No. 4,616,155 issued Oct. 7, 1986 to Guichard and assigned to CITEL Company of France, shows an overvoltage discharger for coaxial cables. The Guichard '155 patent shows an outer conductor forming a bore. A second conductor body is retained within the first conductor and is suspended therein by a pair of rods attached to opposite ends of the second conductor. The rods are positioned generally coaxial with the first conductor by means of insulating members which seal open ends of the first conductor. The first conductor and the insulating seals over the opposite ends thereof define a chamber which house the second electrode therein.
The device as shown as Guichard '155 has many problems in that it is impractical for efficient and large scale use of such a protector. While Guichard '155 appears to have appreciated some of the benefits of a serially mounted protector, it does not provide any solutions for employing such a protector. More specifically, Guichard '155 specifically calls for a first and second rod which are attached to the second electrode. The structure may be unreliable and subject to vibrational or impact effects. Additionally, the connection between the conductor in a coaxial cable and the rods as shown in Guichard is achieved by solder connection. This clearly is impractical for wide use. A more desirable connection would be a mechanical connection.
A device overcoming the problems of the prior art as set forth hereinabove is unknown. As such, it would be highly desirable to provide a serially mounted surge arrester for use with a coaxial cable which can be easily connected to both the central conductor of the coaxial cable as well as the radially spaced outer shield conductor.